ECG Axis Interpretation

Axis

Definition

• the mean direction of electrical forces in the frontal plane ( limb leads) as measured from the zero reference point (lead 1)
• Normal values
  • P wave: 0 to 75 degrees
  • QRS complex: -30 to 90 degress
  • T wave: QRS-T angle <45 degrees frontal or <60 degrees precordial

Measurement

Quick look tests

• The simplest method of identifying gross deviations in axis is to look at the QRS complexes in leads I and aVF. Lead I is a left-sided lead, and as aVF is perpendicular to lead I, it can be considered a right-sided lead.
  • Both leads I and aVF have mainly positive QRS complexes = normal axis.
  • Lead I is positive and aVF is negative = left axis deviation (LAD).
  • Lead I is negative and aVF is positive = right axis deviation (RAD).
  • Both leads negative = extreme RAD or “North-West” axis

• Quick look test — Limb leads

Show axis table

 

Measuring ECG axis (simple version 1)

• Quick look test — leads I and aVF

Show axis table

Measuring ECG axis (simple version 2)

Isoelectric lead method

• identify the most isoelectric lead (contains QRS complexes with equal positive and negative deflections)
• the hearts electrical forces are moving in a direction perpenicular to the isoelectric lead.
• the axis approximates the direction of the lead perpendicular to the isoelectirc lead if the the QRS deflection is positive in the perpendicular lead.
• e.g. if lead I is isoelectric (0 degrees) look at aVF (90 degrees). If the QRS complexes are positive in aVF then the axis is approximately 90 degrees.

Interpretation of QRS Axis

• Normal
  • 0 to 90 degrees
    
• Right Axis Deviation (RAD)
  • > 90 degrees
    • moderate RAD: 90 to 120 degrees
    • marked RAD: 120 to 180 degrees
  • Differential diagnosis
    • Right Ventricular Hypertrophy (RVH) — most common
    • Left Posterior Fascicular Block (LPFB) — diagnosis of exclusion
    • Lateral and apical MI
    • Acute Right Heart Strain, e.g. acute lung disease such as pulmonary embolus
    • Chronic lung disease, e.g. COPD
    • Dextrocardia
    • Ventricular pre-excitation (WPW) — LV free wall accessory pathway
    • Ventricular ectopy
    • Hyperkalemia
    • Sodium-channel blockade, e.g. tricyclic toxicity
    • Secundum ASD — rSR’ pattern
    • Normal in infants and children
    • Normal young or slender adults with a horizontally positioned heart can also
      demonstrate a rightward QRS axis on the ECG.
• Left Axis Deviation (LAD)
  • <-30 degrees
    • moderate LAD: -30 to -45 degrees
    • marked LAD: -45 to -90 degrees
  • Differential diagnosis

• • • Left ventricular hypertrophy (LVH)
    • Left Anterior Fascicular Block (LAFB) — diagnosis of exclusion
    • LBBB
    • inferior MI
    • ventricular ectopy
    • paced beats
    • Ventricular pre-excitation (WPW)
    • Primum ASD — rSR’ pattern
• Extreme Axis Deviation
  • 180 to -90 degrees
    • rare
  • Differential diagnosis
    • Right Ventricular Hypertrophy (RVH)
    • Apical MI
    • VT
    • Hyperkalemia

Rotation

• can be thought of as the axis of the heart in the transverse axis (the precordial leads)
• Normal
  • isoelectric QRS in V3 and V4, indicating the transition point between the right and left ventricular electric forces
• Clockwise rotation
  • isoelectric QRS in V5, V6
• Anti-clockwise rotation
  • isoelectric QRS in V1, V2

Rule of thumb: the heart rotates towards hypertropy and away from infarction